Press forming method and press forming tool

Abstract

A press forming method according to the present invention includes: a first forming step of forming the press formed part by using the first press forming tool having a line length in the cross-sectional direction of the joining portion contacting with a flange portion subjected to stretch flange deformation or a flange portion subjected to shrink flange deformation is a line length L.sub.1 in the cross-sectional direction that is shorter than a line length L.sub.2, where the line length L.sub.2 is a line length in a cross-sectional direction of the joining portion of the second press forming tool for making the product shape; and a second forming step of forming the press formed part in the product shape by crash forming by using the second press forming tool in which the line length in the cross-sectional direction of the joining portion is L.sub.2.

Claims

1. A press forming method for forming a press formed part having a product shape that has a groove-shaped portion extending in a longitudinal direction and includes a flange portion curved along the longitudinal direction on at least one of a pair of side wall portions defining the groove-shaped portion, using a first press forming tool and a second press forming tool each including a side wall forming portion, a flange forming portion, and a joining portion that joins the flange forming portion to the side wall forming portion, the press forming method comprising: a first forming process of forming the press formed part in an intermediate shape by using the first press forming tool having a line length L.sub.1 which is a line length in the cross-sectional direction of the joining portion contacting with a flange portion of the intermediate shape which is subjected to stretch flange deformation or a flange portion of the intermediate shape which is subjected to shrink flange deformation, where the line length L.sub.1 is shorter than a line length L.sub.2, where the line length L.sub.2 is a line length in a cross-sectional direction of the joining portion of the second press forming tool for making the product shape, in order to make a line length of the flange portion of the intermediate shape which is subjected to the stretch flange deformation in a longitudinal direction longer than a line length of the flange portion of the product shape or to make a line length of the flange portion of the intermediate shape which is subjected to the shrink flange deformation in the longitudinal direction shorter than the line length of the flange portion of the product shape; and a second forming process of forming the press formed part from the intermediate shape into the product shape by crash forming by using the second press forming tool in which the line length in the cross-sectional direction of the joining portion is L.sub.2.

2. The press forming method according to claim 1, wherein the first forming process and the second forming process are applied to one of the pair of side wall portions.

3. The press forming method according to claim 1, wherein the first forming process and the second forming process are applied to both of the pair of side wall portions.

4. The press forming method according to claim 1, further comprising forming punch bottom portion of the press formed part by performing the first forming process and the second forming process with a portion of the press formed part corresponding to the punch bottom portion pressed with a pad.

5. The press forming method according to claim 1, wherein the joining portions of the first press forming tool and the second press forming tool have an arc cross-sectional shape.

6. The press forming method according to claim 1, wherein the joining portion of the first press forming tool has an arc cross-sectional shape, and the joining portion of the second press forming tool has a chamfered cross-sectional shape produced by chamfering an arc shape.

7. A press forming tool used for a press forming method for forming a press formed part having a product shape that has a groove-shaped portion extending in a longitudinal direction and includes a flange portion that is curved along the longitudinal direction on at least one of a pair of side wall portions defining the groove-shaped portion and subjected to stretch flange deformation and/or shrink flange deformation, through a first forming process and a second forming process, the press forming tool comprising: a first press forming tool for the first forming process and a second press forming tool for the second forming process, wherein the first press forming tool and the second press forming tool each include a side wall forming portion, a flange forming portion, and a joining portion that joins the flange forming portion to the side wall forming portion, and a line length in a cross-sectional direction of the joining portion of the first press forming tool is set to be shorter than a line length in a cross-sectional direction of the joining portion of the second press forming tool, the joining portion of the first press forming tool being in contact with a flange portion subjected to stretch flange deformation or a flange portion subjected to shrink flange deformation in the first press forming tool.

8. The press forming tool according to claim 7, wherein the joining portions of the first press forming tool and the second press forming tool have an arc cross-sectional shape.

9. The press forming tool according to claim 7, wherein the joining portion of the first press forming tool has an arc cross-sectional shape, and the joining portion of the second press forming tool has a chamfered cross-sectional shape produced by chamfering an arc shape.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view of segment of a press forming tool used for a press forming method according to an embodiment of the present invention.

(2) FIG. 2 is an explanatory diagram of the press forming method according to an embodiment of the present invention, and illustrates a behavior of a blank inside a curve in a forming process.

(3) FIG. 3 is an explanatory diagram of the press forming method according to an embodiment of the present invention, and illustrates a behavior of a blank outside a curve in the forming process.

(4) FIG. 4 is an explanatory diagram of a mechanism by which an effect of the press forming method according to an embodiment of the present invention is produced.

(5) FIG. 5 is an explanatory diagram of a mechanism by which an effect of the press forming method according to an embodiment of the present invention is produced.

(6) FIG. 6 is a cross-sectional view of segment of the press forming tool according to another aspect, the press forming tool being used for the press forming method according to an embodiment of the present invention.

(7) FIG. 7 is an explanatory diagram for explaining the behavior of the blank in the forming process in a case of using the press forming tool illustrated in FIG. 6.

(8) FIG. 8 is an explanatory diagram for explaining a cross-sectional shape of a press formed part to which the present invention can be applied.

(9) FIG. 9 is a diagram illustrating an example of a product shape to which the present invention can be applied.

(10) FIG. 10 is an explanatory diagram of another aspect of the press forming method to which the present invention can be applied.

(11) FIG. 11 is an explanatory diagram of the press forming tool used for another aspect of the press forming method to which the present invention can be applied.

(12) FIG. 12 is an explanatory diagram of the press forming method using the press forming tool illustrated in FIG. 11.

(13) FIG. 13 is a perspective view for explaining a product shape of a press formed part according to an example of the present invention.

(14) FIG. 14 is a cross-sectional view of the press formed part illustrated in FIG. 13.

(15) FIG. 15 is an explanatory diagram of a press forming tool used in Examples 1 to 3 of the present invention.

(16) FIG. 16 is an explanatory diagram of a method of evaluating an amount of springback according to an example of the present invention.

(17) FIG. 17 is an explanatory diagram of a press forming tool used in Example 4 of the present invention.

(18) FIG. 18 is a diagram for explaining a problem of the present invention, and is a perspective view illustrating an example of a product shape of a press formed part as a target of the present invention.

(19) FIG. 19 is a cross-sectional view of a product illustrated in FIG. 18.

(20) FIG. 20 is a diagram illustrating an example of a press forming tool for forming the press formed part illustrated in FIG. 18.

(21) FIG. 21 is an explanatory diagram of the press forming method using the press forming tool illustrated in FIG. 20.

(22) FIG. 22 is an explanatory diagram of a problem of the present invention, and is an explanatory diagram of a mechanism of generating springback in a press formed part formed by the press forming method in the related art.

(23) FIG. 23 is an explanatory diagram of a problem of the present invention, and is an explanatory diagram of springback in the press formed part formed by the press forming method in the related art.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(24) To solve the above problem, the inventors of the present invention have examined a form of springback generated in a press formed part 31 when the press formed part 31 is crash-formed, the press formed part 31 including a groove-shaped portion 31e that includes a punch bottom portion 31a and a side wall portion 31b, and a flange that is formed with flange portions (an outer flange 31c and an inner flange 31d) and curved along a longitudinal direction as illustrated in FIGS. 18 and 19.

(25) In crash forming in the related art, forming is performed using a tool of crash forming 37 including a die 33 and a punch 35 as illustrated in a perspective view of FIG. 20 by sandwiching a blank 39 between the die 33 and the punch 35 as illustrated in FIG. 21. FIG. 22 is a diagram illustrating an outline of the blank before and after forming. For the outline corresponding to a flange portion (hereinafter, the inner flange 31d) on a side where a curve curvature is large (a side where a radius of curvature is small), a curvature is reduced (the radius of curvature is increased) when the blank flows therein by forming, and a line length is increased (A.sub.0B.sub.0.fwdarw.A.sub.1B.sub.1). That is, the inner flange 31d is subjected to stretch flange deformation, and tensile stress remains in a longitudinal direction at a bottom dead center of forming.

(26) In contrast, for the flange portion (hereinafter, the outer flange 31c) on a side where the curve curvature is small (a side where the radius of curvature is large), the curvature of the outline is increased (the radius of curvature is reduced) when the blank flows therein by forming, and the line length is reduced (C.sub.0D.sub.0.fwdarw.C.sub.1D.sub.1). That is, the outer flange 31c is subjected to shrink flange deformation, and compressive stress remains in the longitudinal direction at the bottom dead center of forming.

(27) The residual stress is elastically recovered at the time of die release, shrink deformation is caused in the inner flange 31d, and stretch deformation is caused in the outer flange 31c. As illustrated in FIG. 23, this results in springback of the part, which will in turn cause bending deformation of the part such that the curve curvature is increased (the radius of curvature is reduced). In FIG. 23, a dashed line indicates a shape before springback, and a solid line indicates a shape after springback.

(28) As described above, in the press formed part 31 including the flange portion (the outer flange 31c and the inner flange 31d) curved in the longitudinal direction, the residual stress in the flange portion is released at the time of die release, resulting in springback, which in turn causes bending deformation of the entire press formed part 31. Consequently, it is very important to reduce residual stress in the flange portion of the press formed part 31 to reduce springback.

(29) Accordingly, the inventors of the present invention have conceived, as the method of reducing the residual stress in the flange portion, of forming in which the line length of the flange portion is largely changed as compared with a product shape in the press forming process and the line length of the flange portion is returned to the product shape thereafter. As a specific means of the forming, the inventors of the present invention have conceived of dividing the press forming process into a plurality of processes, and using a press forming tool including a joining portion having a different shape for each press forming tool for each press forming process, the joining portion joining a side wall forming portion and a flange forming portion.

(30) In the press forming method according to an embodiment of the present invention, the press formed part 31 is formed using a press forming tool having a side wall forming portion, a flange forming portion, and a joining portion that joins the flange forming portion to the side wall forming portion, the press formed part 31 illustrated in FIG. 18 having a product shape that has the groove-shaped portion 31e extending in the longitudinal direction and has the flange portion (the outer flange 31c and the inner flange 31d) curved along the longitudinal direction on at least one of a pair of side wall portions 31b defining the groove-shaped portion 31e. The press forming method includes: a first forming process of forming the flange portion subjected to stretch flange deformation so that the line length in the longitudinal direction is caused to be longer than the line length of the flange portion having the product shape, and forming the flange portion subjected to shrink flange deformation so that the line length in the longitudinal direction is caused to be shorter than the line length of the flange portion having the product shape; and a second forming process in which forming is performed using a press forming tool for making the product shape.

(31) The following describes the shape of the press forming tool for each forming process based on FIG. 1 before describing the forming process. Section (a) in FIG. 1 illustrates a cross section of a die shoulder portion of a first press forming tool 1 for the first forming process, and section (b) in FIG. 1 illustrates a cross section of a die shoulder portion of a second press forming tool 3 for the second forming process. The first press forming tool 1 includes a side wall forming portion 1a forming the side wall portion 31b, a flange forming portion 1b forming the flange portion, and a joining portion 1c joining the side wall forming portion 1a to the flange forming portion 1b. As illustrated in FIG. 1, the die shoulder portion is constituted of segment of the side wall forming portion 1a, segment of the flange forming portion 1b, and the joining portion 1c.

(32) The side wall forming portion 1a is a portion that forms a side wall portion of a hat cross-sectional shape, for example, and includes an inclined or vertical flat surface. The flange forming portion 1b is a portion that forms the flange portion (the outer flange 31c and the inner flange 31d) of the hat cross-sectional shape, and includes a flat surface portion. However, the flange forming portion 1b is based on the product shape, and is not necessarily a horizontal surface. The joining portion 1c is a portion that joins the side wall forming portion la to the flange forming portion 1b, which is a portion between a joining point with respect to the side wall forming portion 1a and a joining point with respect to the flange forming portion 1b. Each of ends F.sub.1 and G.sub.1 of the joining portion 1c is a starting point of a curve. Although the die shoulder portion has been described above, the punch shoulder portion is formed to have a similar shape in the first press forming tool 1.

(33) Similarly to the die shoulder portion of the first press forming tool 1, the die shoulder portion of the second press forming tool 3 includes a side wall forming portion 3a forming the side wall portion, a flange forming portion 3b forming the flange portion, and a joining portion 3c joining the side wall forming portion 3a to the flange forming portion 3b. As described above regarding the first press forming tool 1, the punch shoulder portion is formed to have a shape similar to that of the die shoulder portion also in the second press forming tool 3. The length of the joining portion is different between the die shoulder portion of the first press forming tool 1 and the die shoulder portion of the second press forming tool 3, which is a characteristic of an embodiment of the present invention. The following describes the characteristic in detail.

(34) A line length (F.sub.1G.sub.1) in a cross-sectional direction of the joining portion 1c of the first press forming tool 1 is set to be shorter than a line length (F.sub.2G.sub.2) in the cross-sectional direction of the joining portion 3c of the second press forming tool 3. In other words, a length of a flat part of the side wall forming portion 1a of the first press forming tool 1 is longer than a length of a flat part of the side wall forming portion 3a of the second press forming tool 3. With such a shape, forming can be performed such that the blank is pushed into the inside of the part using the die shoulder portion in the first forming process with the first press forming tool 1, and the pushed blank is pushed back to the outside of the part in the second forming process. A mechanism for preventing springback with such a configuration will be described in detail in the following description about the forming method.

(35) First Forming Process

(36) In the first forming process, for example, crash forming as illustrated in FIG. 21 is performed by using the tool of crash forming 37 including the die 33 and the punch 35 illustrated in FIG. 20. In a bottom dead center state in the first forming process, the blank is caused to be in a state represented by a dashed line in enlarged views of FIGS. 2 and 3. FIG. 2 illustrates an enlarged view of the inside of the curve, and FIG. 3 illustrates an enlarged view of the outside of the curve. As represented by the dashed line in FIGS. 2 and 3, at the bottom dead center in the first forming process, positions of flange ends of the blank 39 are A.sub.1 and C.sub.1.

(37) Second Forming Process

(38) In the second forming process performed by using the second press forming tool 3, the blank is pushed back toward the outside of the part (represented by the thick arrow in FIGS. 2 and 3) because a pushing amount of the second press forming tool 3 toward the inside is smaller than that of the first press forming tool 1. As a result, the position of the flange end of the blank 39 is moved to the outside of the part of the flange portion having small restriction. At the bottom dead center in the second forming process, the blank is caused to be in a state represented by the solid line in the enlarged views of FIGS. 2 and 3, and the positions of the flange ends of the blank 39 are A.sub.2 and C.sub.2.

(39) As described above, the positions of the flange ends of the blank 39 are moved by e from the positions A.sub.1 and C.sub.1 at the bottom dead center in the first forming process to the respective positions A.sub.2 and C.sub.2 at the bottom dead center in the second forming process. The following describes a mechanism by which, in a curved part, the flange end is moved to the inside or the outside of the curve based on FIG. 4.

(40) Inside of Curve of Formed Part

(41) At the bottom dead center in the first forming process inside the curve, with reference to an enlarged view of the inside of the curve in a plan view of FIG. 4, A.sub.0B.sub.0 at an inner end 39a becomes A.sub.1B.sub.1 due to entry of the blank 39 from when forming is started to the bottom dead center in the first forming process (first forming process), and the line length of the inner end 39a is increased (stretch flange deformation). At the bottom dead center in the second forming process, the blank is formed into the product shape with the second press forming tool 3 illustrated in FIG. 1. With reference to an enlarged view of the inside of the curve in FIG. 4, at the bottom dead center in the second forming process, the inner end 39a is moved by e toward the inside of the curve, so that the line length of the inner end 39a is changed from A.sub.1B.sub.1 to A.sub.2B.sub.2, that is, the line length is slightly reduced.

(42) Outside of Curve of Formed Part

(43) At the bottom dead center in the first forming process outside the curve, as illustrated in the enlarged view of FIG. 4, C.sub.0D.sub.0 at an outer end 39b becomes C.sub.1D.sub.1 due to entry of the blank 39, and the line length of the outer end 39b is reduced (shrink flange deformation).

(44) At the bottom dead center in the second forming process, the blank is formed into the product shape with the second press forming tool 3 illustrated in FIG. 1. With reference to the enlarged view of the outside of the curve in FIG. 4, the outer end 39b is moved by e toward the outside of the curve, so that the line length of the outer end 39b is changed from C.sub.1D.sub.1 to C.sub.2D.sub.2, that is, the line length is slightly increased.

(45) In this way, the inner flange 31d is formed so that the line length is prolonged as compared with the product shape of the press formed part 31 in the first forming process, and the prolonged line length is slightly restored in the second forming process to be the line length of the product shape of the press formed part 31. On the other hand, the outer flange portion 31c is formed so that the line length is shortened as compared with the product shape of the press formed part 31 in the first forming process, and the shortened line length is slightly restored in the second forming process to be the line length of the product shape of the press formed part 31. Thus, in the inner flange 31d and the outer flange 31c, a strain generated in the first forming process is slightly restored in the second forming process, and the residual stress is significantly reduced accordingly.

(46) Such a configuration will be described below based on FIG. 5. FIG. 5 is a stress-strain diagram in the longitudinal direction after the flange portion is started to be formed. As illustrated in FIG. 5, large residual stress is accumulated in the flange portion at the bottom dead center in the first forming process. However, the residual stress is significantly reduced by slightly restoring the strain in the second forming process from the bottom dead center in the first forming process. As described above, the present invention is made by utilizing a characteristic that the residual stress is largely changed when the strain is slightly restored, that is, the residual stress is sensitively changed in accordance with restoration of the strain.

(47) A restoration amount of the strain is determined based on the movement amount e of the flange end between the first forming process and the second forming process, and the movement amount e is determined based on the shape of the die shoulder portion of the first press forming tool 1 and the second press forming tool 3, specifically, the shape of the joining portion thereof. When a difference in the line length between the joining portions 1c and 3c of the first press forming tool 1 and the second press forming tool 3 is large, the movement amount e of the flange end is large, and the restoration amount of the strain in the longitudinal direction of the part is also large, which leads to a large effect of reducing the residual stress.

(48) In this way, according to the present embodiment, the restoration amount of the strain can be adjusted only by adjusting the shape of the joining portion having a shape of the press forming tool, and springback can be relieved without greatly changing the shape of the press forming tool.

(49) In the above example, the joining portions 1c and 3c having an arc shape are exemplified for both of the first press forming tool 1 and the second press forming tool 3, but the present invention is not limited thereto. It is sufficient that the length of the joining portion 3c in the second press forming tool 3 is longer than the joining portion 1c in the first press forming tool 1. For example, as illustrated in FIG. 6, although the joining portion 1c of the first press forming tool 1 has an arc shape, a joining portion 5c of a second press forming tool 5 has a chamfered shape produced by chamfering the die shoulder portion of the first press forming tool 1. In FIG. 6, the same portion as that in FIG. 1 is denoted by the same reference numeral.

(50) When the blank 39 is formed with the second press forming tool 5 in section (b) in FIG. 6 after being formed with the first press forming tool 1 in section (a) in FIG. 6, the blank 39 is caused to be in a state represented by the solid line from a state represented by the dashed line as illustrated in FIG. 7, the flange end is moved by e toward the outside of the part, and springback can be reduced by the same mechanism as that described above in FIGS. 4 and 5.

(51) The product shape of the press formed part for providing the effect of the present invention may be a shape having a flange portion curved along the longitudinal direction and having the flange portion on at least one of a pair of side wall portions defining a groove-shaped portion. FIG. 8 illustrates a plurality of examples of cross sections of the product shape of the press formed part to which the present invention can be applied, and the following describes each of the cross sections.

(52) Sections (a) to (f) in FIG. 8 include curved flange portions on both of the inside and the outside. The side wall portion may be vertical as illustrated in sections (a) and (d) in FIG. 8, or may be inclined as illustrated in sections (b), (c), (e) and (f) in FIG. 8. As illustrated in sections (c) and (f) in FIG. 8, a shape including no punch bottom portion may be employed, the punch bottom portion being made by connecting both side wall portions at the top. As illustrated in sections (g) to (i) in FIG. 8, the curved flange portion may be provided on any one of the side wall portions. The widths of the right and left flange portions may be different from each other.

(53) With reference to a press formed part 7 in section (a) in FIG. 9 and a press formed part 9 in section (b) in FIG. 9, the curved flange portion may be provided on any one of the inside and the outside, and a non-curved flange portion may be provided on the other one thereof. The entire product shape of the press formed part is not necessarily curved.

(54) The present invention can also be applied to crash forming with a tool of crash forming 21 with a pad using a pad 19 paired with the punch bottom portion as illustrated in FIG. 10. In FIG. 10, a portion that is the same as or corresponding to that in FIG. 21 is denoted by the same reference numeral. In the first forming process, a tool of draw forming 29 including a punch 23, a die 25, and a blank holder 27 illustrated in FIG. 11 can be applied to draw forming including a forming process as illustrated in FIG. 12.

(55) When the curved flange portion is provided to both side wall portions, the effect of the present invention can be exhibited by applying the present invention to only one of the flange portions. The present invention is different from the method disclosed in Patent Literature 1 for preventing springback on a (two-dimensional) cross section such as angle variation of a bending portion of a hat-shaped cross section or curl of a side wall portion. The present invention prevents curl or torsion from being three-dimensionally generated in the entire formed part, so that the effect for the entire press formed part can be provided by applying the present invention to one of the flange portions. This configuration is demonstrated in examples described later.

EXAMPLE 1

(56) The following describes a specific experiment that has been performed for a working effect of the press forming method according to the present invention. The following summarizes an experiment method. The experiment method is such that forming is performed under a plurality of press forming conditions using a press forming device to compare amounts of springback of the formed press formed parts with each other. The press formed part 31 as a forming target has a shape having a hat cross section and curved along the longitudinal direction as illustrated in FIGS. 13 and 14. The length of the press formed part is 1000 mm, the height of the cross section is 30 mm, the width of the punch bottom portion is 20 mm, the width of each of the inner and outer flanges is 25 mm, a curving radius of curvature at the center of the width of the part is 500 mm, and a bending radius of the die shoulder portion is 10 mm. For a steel sheet, a 980 MPa grade steel sheet having a thickness of 1.2 mm was used. A 10000 kN hydraulic press machine was used for a forming test.

(57) The tool of crash forming is used in this example, and the present invention can be applied to both of the die shoulder portion (inner die shoulder portion) that is in contact with the inner flange and the die shoulder portion (outer die shoulder portion) that is in contact with the outer flange. That is, as illustrated in FIG. 15, the first press forming tool 1 was used in the first forming process, the first press forming tool 1 having a distance L.sub.1 (F.sub.1G.sub.1) of the joining portion 1c of 2.1 mm, 4.2 mm, 6.3 mm, and 8.4 mm and a die shoulder radius R.sub.1 of 2 mm, 4 mm, 6 mm, and 8 mm. The second press forming tool 3 was used in the second forming process, the second press forming tool 3 having the distance L.sub.2 (F.sub.2G.sub.2) of the joining portion 3c of 10.5 mm and the die shoulder radius R.sub.2 of 10 mm. A part formed to have a final shape through one time of press forming using the second press forming tool 3 was assumed to be a comparative example. Crash forming with a pad illustrated in FIG. 10 was also performed. A pad pressure was set to be 500 kN.

(58) The shape of the press formed part after press forming was measured through three-dimensional shape measurement. Subsequently, after alignment was performed on measurement data so that a curved part at the center of the longitudinal direction was aligned with a designed shape on CAD software, calculated was a Y-coordinate difference y between measured shape data and designed shape data at an end of the part illustrated in FIG. 16. This value was used as an indicator for bending deformation due to springback. If y is a positive value, it means that the press formed part is bending-deformed in a direction in which the curving radius of curvature thereof is reduced. If y is a negative value, it means that the press formed part is bending-deformed in a direction in which the curving radius of curvature is increased. Table 1 indicates y of the press formed part formed under each forming condition.

(59) TABLE-US-00001 TABLE 1 First Second forming forming Pad Bending process process (Present/ amount L.sub.1 [mm] L.sub.2 [mm] Absent) y [mm] Comparative 10.5 Absent 6.3 example 1 Example 1 of 8.4 Absent 4.8 present invention Example 2 of 6.3 Absent 2.6 present invention Example 3 of 4.2 Absent 0.3 present invention Example 4 of 2.1 Absent 2.0 present invention Comparative Present 6.4 example 2 Example 5 of 4.2 Present 0.5 present invention

(60) A bending amount y tends to be smaller as the distance L.sub.1 of the joining portion 1c of the first press forming tool 1 is reduced, and reversal of positive and negative was caused under a forming condition without a pad where L.sub.1=2.1 mm. A forming condition having the smallest bending amount y was y=0.3 mm when L.sub.1=4.2 mm without a pad, and springback was significantly reduced as compared with the comparative example. Also in forming (Example 5 of the present invention) using a pad, the bending amount y was 0.5 mm, which was significantly reduced as compared with 6.3 mm in Comparative example 1, and the effect of the present invention was confirmed.

EXAMPLE 2

(61) In Example 1, the present invention was applied to both of the die shoulder portion inside the curve and the die shoulder portion outside the curve. However, in Example 2, the present invention is applied to any one of the die shoulder portions to check the effect of reducing springback. The shape of the press formed part, the steel sheet, and a press machine are the same as those in Example 1. A tool of crash forming was used in a forming test of the present invention. The first forming process used the first press forming tool 1 having the distance L.sub.1 (F.sub.1G.sub.1) of the inner or outer joining portion 1c of the first press forming tool 1 of 2.1 mm, 4.2 mm, 6.3 mm, and 8.4 mm. The second forming process used the second press forming tool 3 having the distance L.sub.2 (F.sub.2.sup.-G.sub.2) of the inner and outer joining portions of 10.5 mm. Similarly to Example 1, an evaluation indicator of springback is the bending amount y. Table 2 indicates y of the press formed part formed under each forming condition.

(62) TABLE-US-00002 TABLE 2 Die shoulder to First Second which present forming forming Bending invention is process process amount applied L.sub.1 [mm] L.sub.2 [mm] y [mm] Comparative 10.5 6.3 example 3 Example 6 of Inside 8.4 5.9 present invention Example 7 of 6.3 4.5 present invention Example 8 of 4.2 2.8 present invention Example 9 of 2.1 0.9 present invention Example 10 of Outside 8.4 6.0 present invention Example 12 of 6.3 4.7 present invention Example 13 of 4.2 3.1 present invention Example 14 of 2.1 1.1 present invention

(63) In both of the inner die shoulder portion and the outer die shoulder portion, the bending amount y tends to be smaller as the distance L.sub.1 of the joining portion 1c of the first press forming tool 1 is reduced. At the inner die shoulder portion, y=0.9 mm was satisfied when the distance L.sub.1 of the joining portion 1c=2.1 mm was satisfied, that is, the bending amount y became the smallest value. At the outer die shoulder portion, y=1.1 mm was satisfied when the distance L.sub.1 of the joining portion 1c=2.1 mm was satisfied, that is, the bending amount became the smallest value. In both cases, springback was significantly reduced as compared with the bending amount of 6.3 mm in the comparative example.

EXAMPLE 3

(64) In Example 1 and Example 2, the first forming process was performed by crash forming. In Example 3, a tool of draw forming illustrated in FIG. 11 and FIG. 12 was used for the first forming process, and the present invention was applied to both of the inner die shoulder portion and the outer die shoulder portion. The second forming process was performed by crash forming. The shape of the press formed part, the steel sheet, and a press forming machine are the same as those in Example 1 and Example 2. The first forming process used the tool of draw forming 29 (refer to FIG. 11) having the distance L.sub.1 (F.sub.1G.sub.1) of the joining portion 1c of 2.1 mm, 4.2 mm, 6.3 mm, and 8.4 mm. The second forming process used the tool of crash forming 37 (refer to FIG. 20) having the distance L.sub.2 (F.sub.2G.sub.2) of the joining portion 3c of 10.5 mm. Similarly to Example 1 and Example 2, the evaluation indicator for springback is the bending amount y. Table 3 indicates y of the press formed part formed under each forming condition.

(65) TABLE-US-00003 TABLE 3 First Second forming forming Pad Bending process process (Present/ amount L.sub.1 [mm] L.sub.2 [mm] Absent) y [mm] Comparative 10.5 Absent 4.2 example 4 Example 15 of 8.4 Absent 3.9 present invention Example 16 of 6.3 Absent 1.9 present invention Example 17 of 4.2 Absent 0.3 present invention Example 18 of 2.1 Absent 2.2 present invention Comparative Present 4.4 example 5 Example 19 of 4.2 Present 0.5 present invention

(66) The bending amount y tends to be smaller as the distance L.sub.1 of the joining portion 1c of the first press forming tool 1 is reduced, and reversal of positive and negative was caused under a forming condition without a pad where L.sub.1=4.2 mm. The forming condition having the smallest bending amount. y was L.sub.1=4.2 mm without a pad, and the bending amount y was 0.3 mm in this case. Accordingly, springback was significantly reduced as compared with 4.2 mm in Comparative example 4. In a case of press forming using a pad (Example 19 of the present invention), the bending amount y was 0.5 mm, which was significantly reduced as compared with 4.2 mm in Comparative example 4, and the effect of the present invention was confirmed.

EXAMPLE 4

(67) Example 1 to Example 3 used the second press forming tool 3 the die shoulder portion of which has an arc shape in the second forming process. Example 4 used the second press forming tool 5 the die shoulder portion of which has a chamfered shape. Crash forming was performed with the steel sheet and the press machine that are the same as those in Example 1. As illustrated in FIG. 17, the first forming process used the first press forming tool 1 having the distance L.sub.1 (F.sub.1G.sub.1) of the joining portion 1c of 4.2 mm. The second forming process used the second press forming tool 5 having the distance L.sub.2 (F.sub.2G.sub.2) of the joining portion 5c of 5.3 mm, 7.1 mm, 8.8 mm, and 10.5 mm, and a chamfering amount C of 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm. The evaluation indicator for springback is the bending amount y similarly to the above examples. Table 4 indicates y of the press formed part formed under each forming condition.

(68) TABLE-US-00004 TABLE 4 First Second forming forming Pad Bending process process (Present/ amount L.sub.1 [mm] L.sub.2 [mm] Absent) y [mm] Comparative 4.2 Absent 7.7 example 6 Example 20 of 5.3 Absent 5.5 present invention Example 21 of 7.1 Absent 3.2 present invention Example 22 of 8.8 Absent 0.9 present invention Example 23 of 10.5 Absent 1.2 present invention Comparative Present 7.7 example 7 Example 24 of 8.8 Present 1.1 present invention

(69) The bending amount y tends to be smaller as the distance L.sub.2 of the joining portion 5c of the second press forming tool 5 is increased, and reversal of positive and negative was caused under a forming condition without a pad where L.sub.2=10.5 mm and C=2.0 mm. The forming condition having the smallest bending amount y was L.sub.2=8.8 mm without a pad, and the bending amount y was 0.9 mm in this case. Accordingly, springback was significantly reduced as compared with the bending amount of 7.7 mm in Comparative example 6. In a case of forming using a pad (Example 24 of the present invention), the bending amount y is 1.1 mm, which is significantly reduced as compared with 7.7 mm in Comparative example 6, and the effect of the present invention was confirmed.

(70) The present invention can provide a press forming method and a press forming tool for reducing three-dimensional springback generated in the entire part such as torsion or bending.

REFERENCE SIGNS LIST

(71) 1 First press forming tool

(72) 1a Side wall forming portion

(73) 1b Flange forming portion

(74) 1c Joining portion

(75) 3 Second press forming tool

(76) 3a Side wall forming portion

(77) 3b Flange forming portion

(78) 3c Joining portion

(79) 5 Second press forming tool

(80) 5a Side wall forming portion

(81) 5b Flange forming portion

(82) 5c Joining portion

(83) 7 and 9 Press formed part

(84) 19 Pad

(85) 21 Tool of crash forming with pad

(86) 23 Punch

(87) 25 Die

(88) 27 Blank holder

(89) 29 Tool of draw forming

(90) 31 Press formed part

(91) 31a Punch bottom portion

(92) 31b Side wall portion

(93) 31c Outer flange

(94) 31d Inner flange

(95) 31e Groove-shaped portion

(96) 33 Die

(97) 35 Punch

(98) 37 Tool of crash forming

(99) 39 Blank

(100) 39a Inner end

(101) 39b Outer end